If You See a 40% Chance of Rain, This Is What It Really Means

What happens when your local television station’s meteorologist tells you that there’s a 40% chance of rain? Rainfall probability is one of the most commonly misinterpreted concepts in weather forecasting. Even professional meteorologists seem to misunderstand it sometimes. Some believe that the chance of rain percentage indicates the likelihood that it’s going to rain at all, while others think it means it’s definitely going to rain, just in a specific percentage of the coverage area. The percent chance of rain that meteorologists use is called the “probability of precipitation.” To determine the probability of precipitation, forecasters multiply their level of confidence that it will rain at least 0.01 inches over some period of time by the percentage of the area that’s expected to get rainfall. That means if a meteorologist is 50% confident that 50% of the area will receive rain over a given period of time, the chance of rain would be 25%. However, if a meteorologist is 100% confident that 25% of the area will receive rain, it would also be a 25% chance. What does this mean for us? Honestly, it probably doesn’t mean all that much. If you’re planning an outdoor event, you’ll probably prepare the same way no matter how you interpret the percentage. If you see 80%, there’s a good chance you’ll postpone your party, and if it’s 20%, you can probably go ahead with it. However, it might help to learn the lingo on the chart below.